For the purpose of cooling a planar-type semiconductor controlled rectifier element, a pair of cooling fins are normally disposed on opposite ends of the element so that both surfaces of the element are cooled by the fins. In a converting device wherein a plurality of such semiconductor controlled rectifier elements are employed, a large number of these cooling devices is required. A typical example of such a cooling device is disclosed in U.S. Pat. No. 3,457,472 and a circuit configuration in which such a cooling device is employed is illustrated in FIG. 1a of the accompanying drawings.
FIG. 1a is an illustration of a prior-art circuit for effecting reversible operation of a d.c. motor. The motor 73 has a field winding 74 and is connected to a converter circuit including thyristors 11 through 16, reverse thyristors 21 through 26, capacitors 31 through 36, resistors 41 through 46, anode reactor elements 51 through 56, fuse elements 61 through 68, a forward gate control circuit 71, a reverse gate control circuit 72 and power source lines U, V and W.
For an individual thryistor, such as thyristor 11, there are provided a pair of cooling fins 81 and 82, more specifically shown in FIG. 1b. The cooling arrangement also includes metal fittings 91 and 92, insulator elements 93 and 94 and a spring 95. Thus, for each thyristor or semiconductor controlled rectifier element, a pair of cooling fins is provided.
These cooling fins are required for proper operation of the circuit since current flow through the circuit in both the forward and reverse directions may take place over intervals of several seconds to several hours, as illustrated in FIGS. 2a and 2b. This means that the utilization factor of the cooling fins is 50%.
It has also been proposed to simultaneously clamp thyristors 11 and 12 by a pair of cooling fins so as to enhance the utilization factor of the cooling fins. However, because of the dimensional tolerance of the thickness of the thyristors, this proposal has not been achieved in practice. As a result, cooling fins are disposed independently for each respective thyristor which necessarily means that a large number of cooling fins are required.